Titanium alloys are useful in the aerospace industry because of their high strength to weight ratios at elevated temperatures. The benefits of achieving minimum weight in aircraft components are so significant that extreme techniques are frequently employed to achieve complex geometries and to reduce section thicknesses of components to the absolute minimum dimension permissible by design standards.
Usually, components which are fabricated from sheet or plate material of uniform thickness will have excess material in low stress regions. However, in the interest of saving weight, components are generally fabricated so that material which is not required for load support in a structure is removed.
Conventional mechanical machining techniques, such as milling, are often used to remove material, but these techniques are labor intensive, and generally require expensive machinery which must be operated by highly skilled personnel.
Chemical removal methods are also frequently employed. An aqueous solution containing various acids and often other additives, dissolves material from the surface of the metal. Hydrofluoric acid (HF) in concentrations up to about 10%, usually in combination with one or more other acids, such as hydrochloric acid (HCl), nitric acid (HNO.sub.3), phosphoric acid (H.sub.3 PO.sub.4), sulfuric acid (H.sub.2 SO.sub.4), and various organic acids, in aqueous solution, is commonly used for the chemical milling of titanium and its alloys. HF concentrations greater than about 10% generally result in hard to control reaction rates, poor surface quality and excessive hydrogen absorption.
It is generally accepted that HF permits attack of titanium alloys by dissolving the passive oxide layer that forms on the metal surface. The HF and HNO.sub.3 dissolve the substrate, and the other additives control the rate and uniformity of metal removal, thus contributing to a process whereby metal can be removed rapidly but uniformly over large areas while attaining good surface quality.
Other factors affecting the rate of chemical reaction and metal removal from the surface include loading of the acid solution by metal removed, and the temperature of the acid bath during the reaction. To ensure uniform attack, the acid solution is generally agitated and the parts are often moved within the acid baths. Control of these factors generally results in closely predictable removal rates which provide accurate dimensional control of the finished article.
The chemical milling of alloys is always accompanied by the generation of hydrogen at the reaction surface and is often accompanied by absorption of hydrogen into the metal. This becomes particularly important in alloys susceptible to hydrogen embrittlement, for example, titanium alloys, where hydrogen absorption can result in a drastic reduction in ductility and fatigue life. Alpha titanium alloys are not particularly susceptible to hydrogen embrittlement, but the addition of alloying elements which stabilize the beta phase in the alpha phase titanium results in beta phase-containing alloys or beta alloys which are increasingly susceptible to hydrogen embrittlement.
Many techniques have been suggested for reducing hydrogen absorption during the chemical milling of titanium. Among these are included control of the concentrations of the various acids, and the addition of chromate ions, wetting agents, carbonic acid derivatives or chlorates. U.S. Pat. No. 3,846,188 describes a heat treatment applied to the titanium alloy prior to chemical milling which was shown to reduce hydrogen absorption.
While these techniques have been shown to reduce hydrogen absorption in some situations, they have proven ineffective in protecting certain titanium alloys which require acid solutions with greater than 10% HF for adequate chemical milling rates.
An objective of this invention is to provide a method for the chemical milling of metal alloys which removes metal rapidly while minimizing hydrogen absorption in the metal. As used herein, all references to percentages are to volume percentages, unless otherwise noted.